Two-way television transmission



Jan. 12, 1943. P. MERTZ TWO-WAY TELEVISION TRANSMISSION 4 Sheets-Sheet 2 Filed Dec. 29, 1939 //v VENTOR By P. MEI? TZ A Tram/Er Jan. 12, 1943. P. MERTZ TWO-WAY TELEVISION TRANSMISSION Filed Dec. 29, 1959 4 Sheets-Sheet 5 RNE V M/VENTOR By P. MERTZ w ATT Patented Jan. 12, 1943 TWO-WAY TELEVISION TRANSMISSION Pierre Mertz, Bellerose, N. Y., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a c6rporation of New York Application December 29, 1939, Serial No. 311,490

6 Claims.

This invention relates to signaling systems and method and particularly to a system for two-way transmission of television signals over-,a single transmission channel including, one or more re- I peaters remote from the terminal stations.

An object of the invention is to provide anovel form of signaling system in which the transmission is alternately in opposite directions.

Another'object is to provide novel means for conditioning a repeater in a signaling system to causethe direction of transmission through the repeater to be reversed.

In accordance with a specific embodiment of the invention herein shown and described for the purpose of illustration, there are provided a plurality of terminal stations, each including television transmitting and receiving apparatus, connected by a television transmission line including one or more repeaters. The television transmitting apparatus ateach station generates a television signal which is impressed upon a. television transmission line during intermittent periods of substantially equal duration, alternate line scanning periods for example, and these transmission periods are separated by substan--' tially equal periods during which the signal transmission is interrupted and a television image produced under control of a television signal received over the television transmission line. The transmission periods at the different terminal stations may or may not be concurrent, that is, the signal transmission from the difierentterminal stations may occur simultaneously or the signals may be transmitted from one station while signals are being received at another station. One or more repeaters are provided onthe television transmission lineat a location or locations where the television signals arriving from the different terminals, respectively, are non-overlapping, that is, the position of each repeater on the line is such that the signal propagation time from either terminal to the repeater is substantially equal to the signal transmission period'or to anintegral multiple thereof.

Each repeater on the transmission line (the When there are six repeaters, numbered 1 to 6, inclusive, on the line, for example, so spaced that the'transmission time between repeaters isequal to the .duration of each signal transmission period, the odd numbered repeaters l, 3 and 5 will be. transmitting in onev direction at the same time that the even numbered repeaters 2, 4 and -6 will be transmitting in the opposite direction. However, the repeaters may be spaced anintegral multiple of the distance just assumed,'that v is, by a distance such that the signal transmission time between adjacent repeaters will be an integral multiple of the signal transmission period. If, for example, the odd numbered repeaters 1, 3 and 5 are omitted the even numbered repeaters 2,-'4 and 6 will transmit in one direction during the same time interval and in I the opposite direction during the next timeinterval. If. repeaters numbered 1, 2, 4 and 5 are omitted, the remaining repeaters 3 and 6 will transmit in opposite directions during a certain time interval. a j

In order that each terminal station may transmit or receive atv alltimes so that there will not occur any loss in signaling time, the transmission time between stations must be equal to the signaling period (or to the substantially equal spacing period), or to an odd multiple thereof if the terminal stations transmit simultaneously and receive simultaneously. If one station transmits while the other receives, the terminal stations. should be spaced so that the transmission time between stations is equal to an even multiple of the signaling period (or of the substantially equal spacing period).

The signal transmission time between adjacent repeaters must be made substantially equal to the signal transmission period (line scanning period, for example) or to an integral multiple thereof in order that the television signals from opposite directions may arrive at a repeater during non-overlapping time intervals. If, however, on account of geographical considerations, for example, the repeaters cannot be so spaced that the signaltransmission time between adjacent repeaters is equal to the signal transmission period or to an integral multipl thereof, the

, distance may be made smaller than required and the transmission time" increased to the correct value by. means of a suitable delay network incorporated in the repeater. In this event it may also be necessary to place a delay network in the lin for transmitting the alternating current for controlling the conditioning of the repeater, since the transmission time of the control current should be the same as that of the signaling current.

The invention will now be described in connection with the accompanying drawings in which:

Figs. 1 and 2 when placed side by side with Fig. 1 at the left constitute a diagrammatic view of the two-way television system in accordance with the present invention;

Fig. 1A is a fragment of a front elevation of the scanning disc used at the terminal stations of the television system shown in Figs. 1 and 2;

Figs. 3 and 4 are diagrams to which reference will be made in explaining the operation of the system; and V Figs. 5, 6 and 7 are diagrammatic views of a transmission line including a plurality of intermediate repeaters which may be employed in the television system shown in Figs; 1 and 2.

Referring now to Figs. 1 and 2, there is disclosed a two-way television system comprising the one set including the apertures being employed to direct light from neonlamp |2 to; the

eye ofobserver '|3 for the production of television l3 passing through the lenses 34 and 35, the apertures 33 and the scanning disc apertures I4. The

spirally arranged apertures l4 are equally spaced images under control of received image current erated at station E and transmitted over line L.

and the other set including the apertures i4 being used to transmit light refiectedfrom'the face of the observer l3 to a photoelectric cell ii for ge'n- I 'eratingan' image current for transmission. In

Fig. 1 the-scanning disc is shown as a sideelevvation partly in' section while Fig. 1A shows a fragment of -afrontelevation of the disc. As

shown in the drawings, theface of the observer is illuminated by-lightfrom'a source IE but, if desired,- light could be directed upon the face of are observer through the apertures of the scanning disc to illuminate the elemental areas thereof one at a time in succession and the reflected light re? ceived by a photoelectric cell or other light sen-' sitive electric device. Moreover, if desired, suitable cathode ray apparatus may be employed for scanning the subject and thereby setting up an image current and for producing television images under control of a received image current.

Alternating current from an oscillator I1 is supplied to a motor speed control circuit 8 which controls the speed of the driving motor IQ for scanning disc III, as disclosed in U. S. Patent 1,999,376, granted to H. M. Stoller, April 30, 1935, for example. Current from alternating current source I! is also supplied to the control circuit 20 at station W for periodically reversing the direction of transmission of amplifier 2| and current from this source is also transmitted through amlifier 22 and over line 323 to the repeater station R where it is transmitted through the dela', network 24 and amplifier 25 and thence over l he 26 to the terminal station E where it is supplied to soeed control apparatus H8. like the apparatus 8 at station W for controlling the speed of the scanning disc driving motor at that station.- At the repeater station R the current from source i1 is supplied to a control circuit 220 for periodically reversing the direction of transmission of amplifier MI and at the terminal station E current from source I! is also supplied to a control circuit I20, like the control circuit 20, for periodically reversing the direction of transmission of an amplifier |2| like the amplifier 2| at station W.

by substantially twice the usual distance with respect to the width of the apertures 33 defining the field to be scanned so that the scanning of the field of view is interrupted for a period equal to the line scanning period between the scanning of successive lines. The centers of the apertures II and H are on different radii of the disc l0, respectively, such that the angle formed by the radii going through the centers of two ad: jacent apertures I is bisected by the radius going through the center of an aperture l4. Thus the scanning of a field of view for television transmission and the scanning of the image field for television image production take place alternately. If the line scanning period is t, the subject l3 will be scanned during a perlodt for generating an image current in the circuit of photoelectric cell l5 and, during'the following line scanning period t t, the lamp l2 will be scanned for setting up an image ;under control of the image current gen- The image current generated in the circuit of photoelectric cell i! is amplified by the amplifier 40, the output of which is connected to the primary winding 4| of a transformer, the secondary winding 42 of which is connected to the gridcathode circuit of a four-element vacuum tube 43, one terminal of winding 42 being connected of tube 43 may be traced from the anode 41, winding 48 of a transformer having two otherlwindings 49 and 50, anode battery 9| to the grounded cathode 46, the winding 49 being connected to the line L over which the amplified image current generated by photoelectric cell l5 is transmitted to the repeating station R.

The image current received over line L from the repeater station R is transmitted through transformer windings 49, 5D to the control circuit of amplifier tube 5| which circuit may be traced from grid 52, transformer winding 50, grid biasing battery 53 to the grounded cathode 54. The anode-cathode circuit of tube lil may be traced from the anode 55 through the primary winding 56 of a transformer and anode battery 51 to-the-grounded cathode 54, the secondary transformer winding 58 being connected to the input of amplifier 59. The received image current is thus amplified by the amplifier tube SI and by the amplifier 59 and then impressed upon the image producing lamp l2 which is connected to the output of amplifier 59.

The system is designated so that th image signals from amplifier 40 which are to be transmitted over L, L' to station E and the image sig nals transmitted over line L, L from station E to stationW are alternately impressed upon the amplifier 2|. The control circuit 20 is provided for reversing the direction of transmission of amplifier 2| in synchronism with the transmission and reception of thesignaling impulses .at

' station W. If t is the time required for scan-' a circuit including amplifier tube 43 for a period t and for the next period t signals will be re-'- to zeroora: low value and thus prevent the.

'tube from functioning as an amplifier.

ceived over line L from station E, these signals 1 being amplified by the vacuum tube il. The period of the sine wave from so'urcel'l will then be BI in parallel. The transformer having the pri-.

mary winding 60 has a secondary winding 62 connected to the control grid-cathode circuit of vacuum tube 63 in series with a grid biasing battery 64, the cathode of this vacuum tube being grounded. The anode circuit of vacuum tube 63 comprises an anode battery 65 and an anode resistor 6B, and the anode is connected through a condenser 61 to the grid 68 of vacuum tube 5|.

The average potential of the conductor between condenser 61 and grind 68 is fixed by means of a grid leak or high resistance 69 and a grid biasing 4 battery 10. Similarly, the transformer having the primary winding 6| has a secondary winding 12 connected to the control grid-cathode circuit of vacuum tube 13 in series with a grid biasing battery 14, the cathode of this vacuum tube being grounded. The anode circuit of vacuum tube 13 comprises anode battery 15 and an anode resistor 16, and the anode is connected through a condenser "to the grid '18 of vacuum tube 43. The conductor between condenser I1 and grid 18 r is connected toground through the high resist-:- ance l9 and the grid biasing battery 80.- :The

T the polarity of the electromotive forc from this a resultan'egative impulseis impressed upon the grid, 13 to reduce the anode current of tube 43 Atthe same time that the polarity of the electromotive -'force from oscillator l1 impressedupon the grid of tube 13 changes from negative to positive,

source "impressed upon the -grid of tube 63 changes from positive to negative, thus setting up a positive impulse at the anode of tube 63 which is impressed upon the grid 68 of tube 5! to condition it for amplifying the television signal upon the receiving amplifier 59. It is clear, therefore, that while a line of the field of view comprising the subject l3 isbeing scanned for a time interval t,.the image current generated is repeated by the tube 43 of the terminal repeater 2| and impressed upon the line L. During the next period t, the scanning of the field of view comprising the subject I3 is interrupted and the television signal corresponding to the scanning of a line of the field of view at station E received overline L is repeated by the tube 5| of the terminal repeater 2| andutilized to control the illumination produced by the lamp l2 which is scanned by an aperture I I to producea line of the image viewed by the observer l3.

' of station W except that, at station E, the first digit .1 i addedt'o the numerals used at station W. The control'circuit 220 at the repeater station R is likewise identical with thecontrol cir-, cuit 20 at station Wand the reversible amplifier 22! at the repeater station R is the same as the amplifier 2l alt-{station W. except that there is employedra single tran'sformer winding 258 the terminals of--'which{are-connected to line L'. .instead-of the windings. 58 and of separate transwindings 62 and 12 are so connected to the gridcathode circuits of tubes 63 and 13 that thesine wave voltage from oscillator l1 impressed upon the grid of tube 63, with respect to ground, is 180- degrees out of phase withrespect to that ,im-

pressed upon the grid of tube 13. 'For proper operation, moreover, it is desirable that each of tubes 63 and 13 should be considerably overloaded by the electromotive force from oscillator l1 in order that the anode of the tube will quickly reach a maximum positiveyalue when the tential is impressed upon one plate of condenser 11, the other plate of which is connected to the grid 18 of tube 43. This positive impulse impressed upon the grid 18 conditions the tube 43 to amplify the televison electromotive force from formers, the; terminals of which a rejconnected to amplifiers- 59 and 10,. respectively. With thisj exception- "the-- parts of, the control circuit and. reversible amplifier OITtIieLrepeater R are the Y same as the corresponding parts ofthe' apparatus the transmitting amplifier 40 and the television signal is thus impressed upon line L for transto positive, the potential of the anode of tube 13 is quickly reduced to its minimum value and as at station'W' and aredesigfiatedby numerals which are the same except that, at repeater station R, the first digit 2 is added to the numerals used at station W. There is also associated with the amplifier 22! at repeater station R a delay network 280 which it will be necessary to use to supplement the delay of line L if, because of geographical considerations, the, delay introduced by the line L is less than t, the duration of the signal transmission period at one station which occurs between two signal reception periods each of the same duration t. No delay network is shown in line L, it being assumed in this case thatlthe transmission time of signals transmitted over line L is equal to t, the signal transmission period. A delay network 24 is employed in the control line 23 to delay the control ignal from source I! by the same amount as the television signal is delayed by the delay network'280, as-

suming, of course, that the television signal and the control signal are delayed. by the same amounts in transmission over lines L and 23. respectively. If these delays in transmission are not the same then the network 24 will not have the same delay as network 280 but the overall delay in each circuit will nevertheless be adjusted to be equal to the signal transmission period t. No delay network is shown in control line 26, it being assumed that thetransmission time over this line is equal to that over line L. If the delay in line 26 should be less than that in line L, however, a network similar to network 24 could be inserted to make the over-all value the same. I

If desired, of course, the control signal from source I! may be transmitted over a carrier channel having a frequency outside of the frequency range employed for television transmission and in thiscase a single physical transmission line may be used for transmitting both the television signal and the control signal.

In order to explain the operation of the system, it will be assumed that there aretwo ter minal stations W and E connected by a line L, L having only repeating station R and that the transmitting time from station W to the repeater R, the transmission time from station E to repeater R and the signal transmission period t (the time required for scanning a single line of a field of view, for example) are equal. In this case as will be apparent from the diagram of Fig. 3, signal transmission from terminal stations W and E takes place during alternate periods, that is, the phase relationship of the scanning discs Ill and III] is such that while an aperture I4 at station W i effective for scanning a line of the field of view comprising the subject I3 to cause an image current to be generated, an aperture H I at station E is efiective for scanning the lamp H2 to produce a line of the image observed by the observer II3.

As shown in Fig. 3, at zero time (O-t), the generation of a signal W1 (indicated by the dark or shaded arrow) is started at station W. The signal W1 is then transmitted over the line L until, at time l-t, the generation of the signal at station Wis interrupted and the initially generated portion of the signal (indicated by the arrow-head) has reached the intermediate repeater R. At this time the polarity of the substantially sine wave electromotive force from source H (see Fig. 1) impressed upon'the grid of tube 263 at station R changes from positive to negative, thus reducing the anode current of tube 263 and making the anodeof tube 263 relatively more positive with respect to ground. The resulting positive potential impressed upon the grid 266 of amplifier tube 25I causes the amplifier 22I to be conditioned for transmission from W to E. The signal W1 is thus impressed upon line L and transmitted to station E, the initial portion of the signal W1 reaching station E at time 2-t.

At time l-t the generation of signal E1 (indicated by the light or unshaded arrow) is started at station E. This signal is similarly transmitted over line L until, at time 2-t, the generation of the signal at station E is interrupted and the initially generated portion of the signal (indicated by arrow-head) has reached the intermediate repeater R. At this time the polarity of the substantially sine wave electromotive force from source I I impressed upon the grid of tube 213 at station R changes from positive to negative, thus making the anode of tube 213 relatively more positive with respect to ground. The resulting positive potential impressed upon the grid 218 of amplifier tube 243 causes the amplifier 22I to be conditioned for transmission from E to W, thus causing the signal E1 to be impressed upon line L and transmitted 'to station W. At time 2 t, when the repeater R is conditioned for transmission from E to W, the terminal repeater HI at station E is conditioned for transmission in the W to E direction. In other words, at time 2-t the polarity of the control electromotive force transmitted over line 23, 26 and impressed upon the grid of tube I63 changes from positive to negative to cause a positive potential to be impressed upon the grid I63 of tube I5I to cause the received signal W1 to be transmitted to receiving amplifier I59 and to be impressed upon the lamp H2. At the time 2-t, also, the polarity of the control electromotive force from source I1 impressed upon the grid of tube I3 at station W changes from positive to negative to cause a positive potential to be impressed upon the grid 18 of tube 43 to cause the signal W: generated by photoelectric cell I5 and corresponding to the scanning of the second line of the field of view to be transmitted through amplifier 2| and to be impressed upon the line L, At time 2 /2-t, the signal W1 is being received at station E, the signal E1 is being transmitted from E to W through repeater R and signal. W2 is being transmitted from station W. At time 3-t, the transmission of signal Ea corresponding to the scanning of the second line of the field of view at station E'is started and at the same time the initial portion of signal W2 reaches repeater station R and the initial portion of signal E1 reaches terminal station W. At time SVZ-t signal E1 is being received at station W, signal Ea is being transmitted from station E and signal W2 is being transmitted from W to E through the repeater at station R. At time 4-t the transmission of signal W3 from station W is being initiated while the initial portion of signal W1 has just reached station E and the initial portion of signal E2 has just reached the repeater R. Y

Fig. 4 is intended to illustrate the method of controlling the direction of transmission 01 the amplifiers at the terminal and repeater stations. The curve G represents the sine wave from the control source I I which is impressed upon the grids'of the control tubes 63, I3, 263, 213, I63 and I13. It is apparent from the diagram that the period of the sine wave is 2-t where t is the signal transmission time between stations, for example the transmission time from station W to station R, and where t is also the transmission time of each signaling impulse as represented by W'i, W2 or E1.

At zero time (O-t) the electromotive force G impressed upon the grid of tube I3 at station W is changed from positive to negative with the result that'the anode potential of tube I3 increases rapidly to its maximum value as indicated by curve A (the tube 13 being greatly overloaded by the electromotive force G). The amplifier 43 is thus conditioned for transmitting the signal W1 set up in the circuit of the photoelectric cell I5 at station W. At time 1-t the electromotive force G impressed upon the grid of tube 263 is changing from positive to negative and the potential at the anode of tube 263 increases rapidly to its maximum value as shown by curve A. The amplifier 25I is thus conditioned for transmitting signal W1 in a W to E direction. At the same time 1-t the potential applied to the grid of tube I63 is changing from negative to positive (curve G) and therefore the potential applied to the grid of tube I73 is changed from positive to negative. The potential at anode of tube I13 is therefore increased and as a result the amplifier tube I43 is conditioned for transmitting the,

television signalE1 generated at station E in the circuit of photoelectric cell II5.

Similarly, at-time at. the potential applied to If desired, the odd numbered repeaters of Fig.

the grid of tube 13 changes fromcpositiv eto negmay be omitted, thus leaying thef 'evenjnumative to =cause-.the amplifier tube 43 to be conbered repeatersj2, ,ll and} as shownin Fig. 6,

ditioned for: transmitting the television i signal wz repeaters 2 and (jnowbeing the terminal repeatgenerated ,atstation W.- lAtthegsame m :2"t 5 ers. Asf indicatedithse'repeaters' arefseparated.

to give av transmission time between repeaters equallto 2-1;, In this casehit Willbellobserved at det mine s ti s 2 an 1,4 areps a y-.,a' ti n mlssi it 5 2 W ile we l l' mission conditioning theamplifierazfl ion-trans I g time between each terminal repeatertand theintelevisionsignal Eli-in the Etc Wdimction. ,Also termediate rep ater 4 is .2 5 t. lI'he lfevforef allreattime zzt: the potential tapplied to the grid of peaters must transmitflin the same direction sitube. lfi3isrchangingvfrompositiv Sly, 11 ereiore sign l transmission cause amplifiervtube 15L toebe t the one-terminal stationat transmittingetelevision signal at signal reception takes; place including-amplifier I59and'larn In thesame manner attime 7 4 applied to thegrid of tube 1 w a repeaters l land "l, repeatersalv and J being the negative toepositive and therei -theapotential terminal repeaters which are each separated applied to-tthergridhofz-tubeg63is changing from from intermediatez repeaten l to; give a transpositiveto negative.,:1?ositivepotential -is theretime lbetween repeatersl equal tp 3st,

' terminal repeaters, l and 1-;are sepchanging from negativegto positive an fore the potential. appliedto the grid of tocause s-theetelevision p signal E to be a mitted -to the circuit including amplifier 59 nd lamp -.-l-2, .;;A t the repeater station R thapoten tial applied to the grid of tube-26 ere; ransmit inthesame dire .ion implamplifien tube '25 l :Also at; time tial; appliedito @the grid of m e tiveitq qsit t and tential appliedto the grid -of ingl frompositive to nega tive with he the amplifier-tube |43 1SnCOIldltl0 ed mitti-ng the-television: signal 'E. circuit otphotoelectriccellill5 n iIt willihe'observedfrom w andy that asare employed. el while the signalsgenerated at .station Wp l plione transmis ion on a v i.1 iransm ion l ne, uc a =,aicoaxia1 condu or line, .becau s e ofg thehighline attenuation necessitating a large amplitude output at the; re eaters, nd b ca s t -1 wid f n y ;m. k i d cul t ma 1 description ait -ha been assumed e is re s t fed or wi chi th ree actually there maysbe ia little t me ection V of; transmission 7 h v care ofnthe islight time wh ch may rbe lostein switching, the width: of ithe; ap rt 1 a d 3 mm be ade=s1i ht less than ,ejthe spacingbetween the succes- J repeater; jTo-l take l f l Of the sig ali- ,W1,L for example, i Fig. 4, bal a t-sl gh l less anw io f: ha f 1 oval. hez on ro waver -r- The What is claimed is: direction of transmission through the repeaters 1. A two-way television system comprising two at a certain time is indicated by the arrows. It terminal stations each having television transwill be observed from Fig. 5 that repeaters which mitting means'for scanning a field of view along are spaced so that the transmission time of telesuccessive substantially parallel lines to produce vision signals between them is equal to the signal an image current, and television receiving means transmission period t or to an odd integral for building up an image along successive submultiple thereof, are conditioned 'for transmisstantially parallel lines under control of a resion in opposite directions simultaneouslywhile, ceived television image current, the lines of the on the other hand repeaters which are spaced so field of view of the television transmitter and the that the transmission time of signals between lines of the image field of the television receiver them is equal to an even integral multiple of the at one station being scanned during alternate signal transmission period t are conditioned for line scanning periods, a transmission line includtransmission in the same direction simultaneing a plurality of repeaters for transmitting the ously. Since the transmission time between the 0 television current from each terminal station to two terminal repeaters 1 and 6 is equal to 5-t, thev other terminal station, the transmission time these repeaters must be conditioned for transbetween repeaters being equal to a line scanning mission in opposite directions simultaneously and period or to an integral multiple thereof and therefore signal transmission must take place m a for conditioning ea h r p t for t from both terminal stations .at the same time.-- mission in one direction only at a given time and retr sm ssion t mea ghese reneatsive scan lng apertures; This williresultzin the ning periods, repeaters between which the transmission time is equal to a line scanning period or to an odd integral multiple thereof being con-- ditioned for transmission in opposite directions simultaneously and repeaters between which the transmission time is equal to an even integral multiple of a line scanning period being conditioned for transmission in the same direction simultaneously.

2. A two-way television system in accordance with claim 1- in which means are provided for synchronizing the television transmitter scanning means at the two stations in proper phase such that the image current generated at one terminal station will be received at the other terminal station during a period in which the image field of the television receiver at said other station is being scanned.

3. A two-way television system in accordance with claim 1 in which there are provided at one of the terminal stations 9. source of alternating current having a period equal to twice the line scanning period and means for transmitting alternating current from said source to the other terminal station for maintaining the terminal scanning apparatus in such phase relationship that the transmitting scanning apparatus is effectiveionscanning the fieldoi' view at both stations simultaneously when the signal transmission time between terminal stations. is an odd integral multiple of the line scanning period and that 'the transmitting scanning apparatus at one station is effective for scanning the field of view at the same time that the receiving scanning apparatus at the other'station is effective for scanning the image field when the signal transmission time between terminal stations is an even integral multiple of the line scannin period.

4. A two-way television system comprising two terminal stations, a transmission line connecting said terminal stations and including a plurality of repeaters, transmitting and receiving scanning apparatus at each terminal station for scanning a portionof'a field of view and scanning a portion of an image field for producing a television image under control of received image current alternately, control means associated with each repeater for conditioning the repeater for transmission in opposite directions -altemately, a source of alternating current, means under control of alternating current from said source for maintaining the transmitting and receiving scanning at the terminal stations in synchronism and in proper phase relationship, and means under control of alternating current from said source for controlling said repeater control means for causing the reversal of the direction of transmission of eachrepeater' in synchronism with the alternate scanningoi" the field of view for television transmission and of the image field for television reception at one of said stations. a

5. A two-way signaling system comprising means for intermittently transmitting signals at each of a plurality of terminal stations of said system, a signal transmission channel including a repeater, said channel having aninherent delay characteristic such that signals transmitted from different stations,-respect ively, would overlap at said repeater if no means were provided to prevent said overlap, a source of control current for controlling the direction of transmission of said signals over said channel, a second channel for transmitting said control current, and delay networks in said channels respectively for supplementing the inherent delay thereof, thereby preventing overlap at said repeater of signals transmitted from different stations, respectively.

6. A two-way television system comprising two terminal stations each having television trans-'- mitting means for scanning a field of view to produce an image current and television receiving means for scanning-an image field for building up an image under control of a received image current, the field of view of the television transmitter and the image field of the television receiver at one station being scanned during alternate substantially equal time periods, each time period being at least as long as a line scannin period, a transmissionline including a plurality of repeaters for transmitting the television current i'romeach terminal station to the other terminal station, the transmission time between repeaters being equal to said time period or to an integral multiple thereof, and means for conditioning each repeater for transmitting in one direction only for said time period and in opposite directions alternately for equal time periods, repeaters between which the transmission time is equal to said time period or toan odd integral multiple thereof being conditioned for transmission in opposite directions simultaneously and repeaters between which the transmission time is equal to an even integral multiple of said time period being conditionedfor transmission in the same direction simultaneously.

- PIERRE MERTZ. 

